This invention relates to polymers useful as the base polymer in chemical amplification resist compositions suited for microfabrication. It also relates to chemical amplification resist compositions comprising the polymers, and a patterning process using the same.
In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. In particular, the change-over from i-line (365 nm) to shorter wavelength KrF laser (248 nm) brought about a significant innovation, enabling mass-scale production of 0.18 micron rule devices. To the demand for a resist material with a higher resolution and sensitivity, acid-catalyzed chemical amplification positive working resist materials are effective as disclosed in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography.
Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.3 micron process, went through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.15 micron rule, with the trend toward a finer pattern rule being accelerated.
A wavelength change-over from KrF to shorter wavelength ArF laser (193 nm) is expected to enable miniaturization of the design rule to 0.13 xcexcm or less. Since conventionally used novolac resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they cannot be used as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and alicyclic (typically cycloolefin) resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198. With respect to F2 excimer laser (157 nm) which is expected to enable further miniaturization to 0.10 xcexcm or less, more difficulty arises in insuring transparency because it was found that acrylic resins are not transmissive to light at all and those cycloolefin resins having carbonyl bonds have strong absorption. As long as the inventor has confirmed, polyvinylphenol has a window for absorption in proximity to 160 nm, so the transmittance is somewhat improved, but far below the practical level, and reducing carbonyl and carbon-to-carbon double bonds is essential for insuring a transmittance.
An object of the invention is to provide a novel polymer having a high transmittance to vacuum ultraviolet radiation of up to 300 nm, especially of 157 nm, 146 nm, 134 nm and 126 nm, and useful as the base resin in a chemical amplification resist composition. Another object is to provide a chemical amplification resist composition comprising the polymer, and a patterning process using the same.
It has been found that a polymer comprising recurring units of the following general formula (1) is effective as the base polymer in chemical amplification resist compositions because the resulting resist composition has an improved transmittance to vacuum ultraviolet radiation. 
Herein at least one of R1 and R2 is fluorine or a trifluoromethyl group, and the remainder is hydrogen or a straight, branched or cyclic alkyl group of 1 to 20 carbon atoms, R3 and R4 each are hydrogen or a straight, branched or cyclic, unsubstituted or fluorine-substituted alkyl group of 1 to 20 carbon atoms, or R3 and R4, taken together, may form a ring.
In a first aspect, the invention provides a polymer comprising recurring units of formula (1) defined above and recurring units having acid labile groups.
The recurring units having acid labile groups are preferably of any one of the following formulae (2) to (8): 
Herein R5 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 10 carbon atoms, R6 is an acid labile group, and R7 is an alkylene group of 1 to 10 carbon atoms.
In a second aspect, the invention provides a chemical amplification positive resist composition comprising (A) a polymer comprising recurring units of the formula (1) or a polymer comprising recurring units of the formula (1) and recurring units having acid labile groups, (B) an organic solvent, and (C) a photoacid generator. The resist composition may further include (D) a basic compound and/or (E) a dissolution inhibitor.
In a still further aspect, the invention provides a process for forming a pattern, comprising the steps of applying the resist composition onto a substrate to form a coating; heat treating the coating and exposing the coating to high energy radiation with a wavelength of up to 300 nm or electron beam through a photo-mask; optionally heat treating the exposed coating, and developing the coating with a developer.
As long as the inventor knows, polyvinyl alcohol has relatively low absorption because of the absence of carbonyl group, and halogen substitution, especially fluorine substitution on polyvinyl alcohol is effective for improving transmittance to a practically acceptable level. Since the development step of photolithography generally involves applying alkaline water to a resist film by puddling or dipping, rinsing with pure water and spin drying, it is necessary to adjust the solubility of a polymer in alkali relatively high and the solubility in pure water relatively low. Since polyvinyl alcohol is so water soluble that it is used as the base polymer in resists adapted to water development, polyvinyl alcohol has the problem that it is dissolved not only upon alkali development, but also upon rinsing. This necessitates to increase the solubility of a polymer in alkali, thereby enlarging the difference from its solubility in water.
The units represented by formula (1) according to the invention are characterized in that they are cleaved with an acid to create a fluoroalcohol and thus become soluble in alkali. More specifically, the polymer of the invention has units in which dialcohol is blocked with an acetal bond whereby its solubility in alkaline solution is improved over polyvinyl alcohol. By copolymerizing recurring units having acid labile groups, the polymer is improved in dissolution contrast, providing for a better positive resist material. Among the functions required for photoresists, the alkaline solubility is, of course, crucial to form a pattern, but satisfying only the alkaline solubility is insufficient. That is, dry etching resistance and adhesion to the substrate are also crucial functions. To improve the dry etching resistance, it is generally believed effective to increase the carbon density and to incorporate a cyclic structure as typified by a bridged cyclic hydrocarbon group. The polymer of the invention is effective in this respect too.